This invention relates to an open architecture superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called "MRI"), and more particularly to an improved and simplified passive shimming arrangement in such an assembly.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are superconducting, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed due to the absence of resistance, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of MRI.
Considerable research and development efforts have been directed at eliminating the need for a boiling cryogen such as liquid helium. While the use of liquid helium to provide cryogenic temperatures has been widely practiced and is satisfactory for MRI operation, helium is found and commercially obtained only in the state of Texas. As a result, the provision of a steady supply of liquid helium to MRI installations all over the world has proved to be difficult and costly. This has led to considerable effort being directed at superconducting materials and magnet structures which can be rendered superconducting for MRI use at relatively higher temperatures such as ten degrees Kelvin (10K), which can be obtained with conduction cooling.
Another problem encountered by most MRI equipments is that they utilize solenoidal magnets enclosed in cylindrical structures with a central bore opening for patient access. However, in such an arrangement, the patient is practically enclosed in the warm central bore, which can induce claustrophobia in some patients. The desirability of an open architecture MRI magnet in which the patient is not essentially totally enclosed has long been recognized. Unfortunately, an open architecture MRI magnet poses a number of additional and unique technical problems and challenges. One problem is to provide a suitable superconducting structure which will provide the required magnetic field yet occupies much less space than conventional cylindrical MRI magnet structures, and yet which nevertheless can provide the required strong and extremely homogenous magnetic field even though the spaced magnet coils are under considerable electromagnetic forces, plus thermal forces encountered during cool-down from ambient temperature to superconducting temperatures. An open architecture MRI with which the subject invention could be used is disclosed in U.S. patent application Ser. No. 07/970,511, filed Nov. 2, 1992 by Bu-Xin Xu and Olewasequn Ige entitled "Open Architecture Magnetic Resonance Imaging Superconducting Magnet Assembly," assigned to the same assignee as the subject invention, and hereby incorporated by reference.
In order to compensate for the inhomogeneities in MRI magnets, various arrangements including ferromagnetic shim materials have been used. However, the need for magnetic field homogeneity is most critical in the central region of an MRI magnet, the very region in an open MRI magnet which is not available for the placement of effective shimming material adding to the basic problem. The central region of an MRI magnet is the very region which for a variety of technical reasons or problems is the region of interest where the patient imaging must take place.
Moreover, open architecture MRI magnets tend to produce increased field inhomogeneity in this region of interest due to increased coil deformations and coil misalignment. Prior art superconducting magnet designs are directed at minimizing such inhomogeneity during the design stage, and then add passive shim systems to reduce the inhomogeneity that remains after the manufacturing cycle due to manufacturing tolerances and design restrictions. This results in an open architecture MRI magnet that requires shimming in the open area of the magnet between the two superconducting coil assemblies, or on the outer surfaces of the vacuum vessel enclosing the superconducting coil assemblies. The open area is not available for placing shims in an open architecture MRI magnet design. Placing shims on the outer surface of the vacuum vessel is not desirable because the shims are not as effective in that location, requiring that they be thicker such that the forces on the shims are very large.
Furthermore, the outer surface region is frequently not readily accessible at the hospital installation site. In addition, adding passive shim assemblies to the outer surfaces of the vacuum vessel results in considerable additional expense.
Finally, there are a number of other problems, including problems of differential thermal expansion and contraction of materials, of minimizing cost, and of handling the forces generated by the significant magnetic fields required. All of these overlapping and at times conflicting requirements must be satisfied for a practical and satisfactory MRI superconducting magnet structure.